Russia is calling for an independent investigation by UN experts into what Syrian rebels alleged was a chemical weapons attack by government forces on the outskirts of the capital Damascus.

A statement from the Russian Interior Ministry said that minister Sergey Lavrov and US secretary of state John Kerry had discussed the situation by telephone and concluded they had a "mutual interest" in calling for the UN investigation.

It said Russia had called for Syrian president Bashar Assad's embattled government to co-operate with an investigation, but questions remain about the willingness of the opposition, "which must secure safe access of the mission to the location of the incident".

Russia has been one of Assad's key allies in the international arena.

Fierce battles and shelling have been reported in suburbs of Damascus, where the opposition claimed the chemical weapons attack this week killed more than 100 people. In neighbouring Lebanon, the army said that troops have captured a truck with "large amounts" of gas masks in the south-eastern village of Kfeir near the border with Syria.

The offensive has entered its third day and comes as UN secretary-general Ban Ki-moon urged the Syrian government to allow a team now in Damascus to swiftly investigate the alleged attack. Mr Ban is also sending UN disarmament chief Angela Kane to Syria to press for an investigation.

Meanwhile, the number of registered child refugees fleeing Syria's violence has topped the one million mark, two UN agencies have said. The refugee and children's agencies said the grim milestone - half of all the nearly two million registered refugees are children - is not just another statistic.

Anthony Lake, head of the UN children's agency Unicef said in a statement that the millionth child refugee is "a real child ripped from home, maybe even a family, facing horrors we can only begin to comprehend".

Mr Lake and Antonio Guterres, head of the UN refugee agency, said about 7,000 children are among the more than 100,000 killed in the unrest in Syria, which began in March 2011 and later exploded into a civil war.